Modern metal working and forming processes normally require the use of cutting fluids whose function is to facilitate the machining operations by cooling and lubricating. The cooling function is accomplished by the ability of the fluid to carry off the heat generated by the frictional contact between the tool and the work piece and/or any heat resulting from the plastic deformation of the work. Cooling aids tool life, preserves tool hardness and helps to maintain the dimensions of the machined parts. The second function is accomplished by the ability of the fluid to lubricate the tool work piece interface in order to reduce tool wear, frictional heat generation and power consumption. The presence of a fluid also serves to carry away debris from the work area.
In addition to the primary functions of cooling and lubricating, cutting fluids should protect the machined surfaces, tools and other equipment from rust and corrosion, should not themselves corrode, discolor or form deposits in or on the work area, and should not produce undesirable flames or smoke. Additionally, in those instances where skin contact is unavoidable, the cutting fluid compositions should be non-toxic and dermatologically safe.
Moreover, the cutting fluid composition itself should, ideally have some stability against microorganisms. Bacteria and fungi frequently spoil soluble cutting fluids (especially oil and water emulsions) during machining operations. Not surprisingly, the cutting environment, which contains warm water and an available carbon source, provides a good medium for microorganism growth. Spoilage often manifests itself first as a foul smell. This can also be caused by the presence of sulfur-containing additives that are capable of being reduced to hydrogen sulfide, which smells like rotten eggs.
Sulfur-containing additives have been made from treatment of olefins or fats with elemental sulfur. The resulting products are black in color and smell of hydrogen sulfide. Additionally, free thiol functionalities will react with many metals, especially those containing copper, and stain the metal black. Alternative products are polysulfides, which can be light in color but tend to decompose into elemental sulfur and mercaptans upon standing. The polysulfides and the sulfurized fats and olefins are only soluble in oil.
Sulfur-containing additives become functional under conditions of extreme heat and pressure, producing metal sulfides which can improve metal cutting and forming efficiency. Metalworking fluids that incorporate sulfur-containing additives can improve the efficiency of various metalworking operations.